The adsorption of carbon monoxide on H-Faujasite
(H-FAU) and metal-exchanged Li-FAU zeolites has
been investigated by means of both the quantum
cluster and the embedded ab initio cluster approaches.
For the H-FAU/CO complexes, the adsorption energy
of the bare quantum cluster is evaluated to be
21.90 kcal/mol. Inclusion of the Madelung potential,
via the Surface Charge Representation of the Electrostatic
Embedding Potential (SCREEP) method, has a
pronounced influence on the OH distance, hence
increasing the binding energies of H-FAU/CO
(23.20 kcal/mol). The results obtained are in agreement
with experimental data. For adsorption of CO on
the metal-exchanged zeolites, the binding energies of
the complexes obtained from the effect of Madelung
potential can be employed to investigate the different
types of zeolites. For the bare quantum clusters, Li-
ZSM-5/CO and Li-FAU/CO complexes, have almost
the same binding energies (25.77 vs. 25.81 kcal/
mol) while the binding energies derived from the
embedded method of Li-ZSM-5/CO are calculated
to be 28.56 kcal/mol which are larger than those
obtained from the Li-FAU complex (26.69 kcal/
mol), indicating that the ZSM-5 is more acidic than
that of FAU zeolites and leads to a better agreement
with experimental observation. The results obtained
in the present study suggest that the embedded cluster
approach yield a more accurate and practical model
than the bare quantum cluster for exploring zeolite
framework and catalytic properties.
Acknowledgements
This work was supported by donors of the Thailand
Research Fund (TRF) in supporting the research
career development project (The TRF Research
Scholar) and Royal Golden Jubilee PhD Program, as
well as the Kasetsart University Research and Development
Institute (KURDI) to J.L. Our sincere thanks
are due to Professor R. Ahlrichs (Karlsruhe,
Germany) and Professor T.N. Troung (University of
Utah, Salt Lake City, USA) for their continued
support of this work.